This invention relates to light sources, including light emitting diodes, to point to line and point to plane light converters, and to lighting devices, frontlights, backlights and electronic displays.
Various applications require a light source that is extended in one dimension, referred to as a line source. A simple example is the cold-cathode fluorescent tube (CCFT) commonly used in signage, room lighting, and for illuminating the input edge of slab waveguides used for display backlighting or front lighting. To reduce costs and improve energy efficiency, it is desirable to replace CCFT sources with discrete or point-like sources (that is, sources in which the chief rays originate approximately from a point) such as light emitting diodes (LEDs) or small filament incandescent lamps, utilizing methods to spread the light from these sources along a line. Methods for spreading the light include the use of a point to line converter consisting of a bar or cylindrically shaped light guide with prismatic features along the long axis to reflect and redirect light propagating through the light guide, e.g., as in U.S. Pat. Nos. 4,528,617 (Blackington); 5,506,929 (Tai et al. ""929); 5,688,913 (Tai et al. ""913) and 5,835,661 (Tai et al. ""661). Such methods tend to rely on total internal reflection and are often inefficient, losing as much as 50% of the light due to total internal reflection failure. Another method uses diffusive scattering along a line, e.g., by incorporating diffusive features within the light guide to disperse and redirect light propagating through the light guide. These diffusive scattering methods generally suffer from non-uniformity, low efficiency, and coloration resulting from the wavelength dependence of the scattering mechanism. Other point to line converters are shown in U.S. Pat. Nos. 4,751,615; 5,163,748; 5,613,751; and 5,901,266.
Some applications require a light source that is extended in two dimensions, referred to as a plane source. For example, a backlight and point to plane converter for one or more LEDs is shown in U.S. Pat. No. 6,139,163, and a light pipe is shown in U.S. Pat. No. 5,309,544.
Several papers by R. Winston of the University of Chicago and others discuss xe2x80x9cnonimaging opticsxe2x80x9d and the use of nonimaging optics for the design of solar concentrators, see e.g., Proceedings of SPIE Conference 2538, Nonimaging Optics: Maximum Efficiency Light Transfer III, R. Winston, ed. (International Society for Optical Engineering, San Diego, Calif., July 1995). An article by D. Jenkins and R. Winston appearing on the Internet at http://hep.uchicago.edu/solar/light.html and entitled xe2x80x9cNew Reflectors for Illuminationxe2x80x9d discusses the use of nonimaging optics and shaped reflectors for use in illumination.
Existing point to line and point to plane converter devices do not always provide sufficiently even light output, especially when the light source itself has non-uniform or asymmetric light output. Often these converter devices must rely on downstream components (e.g., the backlight or frontlight in a display) to improve the evenness of the light output seen by a viewer of the display.
We have discovered shaped refractive articles that provide substantially even distribution of light from one or more point-like light sources to a target line or target plane. When illuminating a target line, the shaped articles will typically be thin and substantially two-dimensional, and can provide point to line conversion. When illuminating a target plane, the shaped articles will typically be thicker and substantially three-dimensional, and can provide point to plane conversion. The target can be a part of the refractive article or located remotely from the article. We will refer to illumination of a target line or target plane on a refractive article as illumination of a xe2x80x9ctarget aspectxe2x80x9d. We will refer to illumination of a target line or target plane remote from a refractive article as illumination of a xe2x80x9ctarget regionxe2x80x9d. We will use the word xe2x80x9ctargetxe2x80x9d to refer collectively to a target aspect or target region.
In one embodiment, the refractive articles of the invention comprise an input edge or face and an output edge or face, the input edge or face having at least one shaped notch or cavity that can at least partially envelop a light source, wherein the notch or cavity has at least two oppositely signed curved portions shaped to provide substantially uniform illumination of a target by the source. If the notch or cavity is divided into two parts, then each part preferably has a first curved portion near the target that is concave with respect to the light source, and a second curved portion more remote from the target that is convex with respect to the light source, with the second curved portion being shaped to illuminate the target at approximately the same light intensity per unit of target length as the first curved portion.
In another embodiment, the refractive articles of the invention are optically coupled to and at least partially encapsulate a light source, and have an output edge or surface having at least two oppositely signed curved portions shaped to provide substantially uniform illumination of a target region by the source. These latter refractive articles preferably have a first curved portion of the output edge or surface that is convex with respect to the light source, at least partly surrounded by a second curved portion that is concave with respect to the light source, with the second curved portion being shaped to illuminate the target at approximately the same light intensity per unit of target area as the first curved portion.
In another embodiment, the invention provides a method for illuminating a target using a light source, comprising refracting light from the source through a refractive article having an edge or surface having first and second oppositely signed curved portions shaped to provide substantially uniform illumination of the target by the source, wherein the second curved portion is shaped to illuminate the target at approximately the same light intensity per unit of target length as the first curved portion.
In yet another embodiment, the invention provides a method for designing a curved portion of a shaped refractive article to provide substantially uniform illumination of a target by light from a source that is refracted through the curved portion, comprising the step of shaping the curved portion so that it substantially obeys the following equations:             sin      ⁢              xe2x80x83            ⁢              θ        2              =                  (                              n            1                                n            2                          )            ⁢              sin        ⁡                  (                      α            -                          θ              0                                )                                tan      ⁡              (                  α          -                      θ            2                          )              =                            x          f                -                  x          s                            d        -        y                        tan      ⁢              xe2x80x83            ⁢              θ        0              =                  x        s            y                          ⅆ        y                    ⅆ                  x          s                      =                  -        tan            ⁢              xe2x80x83            ⁢      α                          ⅆ                  x          f                            ⅆ                  θ          0                      =                  (                  1          C                )            ⁢                        ⅆ                      I            0                                    ⅆ                      θ            0                              
in a Cartesian coordinate space in which the x-axis is parallel to the target, the y-axis is perpendicular to the target, and the source is at the origin, wherein:
(xs, y) is the position of a point on the curved portion,
xcex82 is the angle of refraction of a light ray at position (xs, y),
n1 is an index of refraction between the source and the curved portion,
n2 is an index of refraction between the curved portion and the target,
xcex1 is the angle of the curved portion with respect to the x-axis at position (xs, y),
xcex80 is the angle made by light rays from the source with respect to the y-axis,
xf is the position of a point on the target along the x-axis,
d is the distance from the source to the target along the y-axis,
dI0/dxcex82 is the distribution of light from the source,
C is the desired illuminance at a point on the target, equal to I00/2xf0 
I00 is the total optical output power of the source and
xf0 is the half-length of the target, measured along the x-axis.
The invention also provides lighting devices, frontlights, backlights and displays comprising one or more of the above-mentioned refractive articles.